Graphene is an atomically thick, two-dimensional sheet composed of sp2 carbons in a honeycomb structure. Graphite (3-D) is made by stacking several layers on top of each other, with an interlayer spacing of ˜3.4 Å.
Graphene oxide (GO), graphene's oxygenated and hydrophilic counterpart, comprises carbon sheets having oxygen functional groups on the sheet basal planes and/or edges. In other words, graphene oxide is oxygenated graphene, which contains C—O bonds where the carbon is spa hybridised and/or C═O bonds where the carbon is sp2 hybridised. It has also attracted much attention in recent years due its potential applications ([Klinowski 1998], [Titelman 2005], [Dikin 2007]). Of particular interest is the fact that GO nano-sheets are soluble in water and have oxygen-containing functional groups on their surfaces ([Klinowski 1998] and [Titelman 2005]). These properties make them very attractive for applications such as energy-related materials, sensors, and bio-applications [Park 2010]. GO is also a promising candidate for the preparation of paper-like materials [Chen 2009]. More importantly, GO is considered as a promising precursor for the large-scale production of graphene-based materials owing to its relatively low cost of synthesis [Chen 2009] and [Stankovich 2007].
Chemical Methods of Preparing Graphene Oxide
The first syntheses of graphene oxide were reported over a century ago. In 1859, Brodie demonstrated the first synthesis of GO by adding potassium chlorate to a slurry of graphite in fuming nitric acid [Brodie 1859]. Staudenmaier later improved this method by adding the chlorate in small portions over the course of the reaction [Staudenmaier 1898]. He also added concentrated sulfuric acid to increase the acidity of the mixture. This slight change in the procedure resulted in an overall increase in the extent of oxidation.
In 1958, Hummers and Offeman introduced the most currently used method to prepare GO [Hummers 1958]. They oxidized graphite with KMnO4 and NaNO3 in concentrated H2SO4.
After oxidizing bulk graphite, GO sheets were exfoliated from the oxidized bulk via sonication in aqueous solution.
A drawback of these oxidative chemical methods, however, is that they need to be conducted over a period of days. These methods also involve the handling of highly reactive acids and oxidants and require extensive work-up of the reaction mixture once the reaction has completed, for example to dispose of the harmful acid and oxidants and to treat the toxic gaseous bi-product(s) NO2, N2O4, and/or ClO2 (the latter also being explosive [Dreyer 2010]). Thus, these methods are not ideal for industrial scale production. In addition, the extensive sonication treatment necessary to drive effective exfoliation of the graphene sheets in these reactions tends to break the GO sheets and therefore limit the flake sizes that may be produced.
Electrochemical Methods of Producing Graphene Oxide
Disclosures of electrochemical methods for producing graphene are well reported in the literature. For example, Liu et al. [Liu 2008] reported the exfoliation of graphite using an ionic liquid-water mixture electrolyte to form “kind of IL-functionalized” graphene nano-sheets. Lu et al. [Lu 2009] showed that the graphene nano-sheet production in Liu's method was at the anode and is due to an interaction of decomposed water species and the anions from the ionic liquid, such as BF4−. The present inventors have also reported in WO2012/120264 a method of producing graphene by the electrochemical insertion of alkylammonium cations in a solvent into graphite. Another electrochemical method for producing graphene has also been published in WO2013/132261 whereby double intercalation of graphite into the negative electrode occurs with metal and organic ions. In these methods, the emphasis is naturally placed on providing non-oxidative conditions to maximise the production of graphene.
However, only more recently have electrochemical methods of producing graphene oxide have been reported. For instance, You et al. [You 2011] describes a two-step process whereby graphite is first expanded by concentrated sulfuric acid to force the graphite crystal lattice planes apart and introduce reactive intercalating ions (e.g. sulphate ions) between the graphite layers. This expanded graphite was then subject to electrochemical exfoliation in an aqueous electrolyte solution of 1M potassium chloride over 10 hours. The authors propose that the chloride ions intercalate in the pre-expanded graphite anode and react to produce chlorine gas bubbles between the graphene layers. The chlorine is proposed to electrochemically oxidise the graphene layers whilst the bubbles provide a crucial additional expansion force to separate the layers leading to exfoliation of GO sheets. It is also probable that the pre-intercalated sulphate ions also contribute to the oxidation and expansion process. This process is conducted over a shorter time-scale compared to the chemical methods described above (hours rather than days). However, this method suffers from similar drawbacks to the conventional chemical oxidation methods described above in that highly reactive materials are used and produced, e.g. concentrated acid is required to first prepare an expanded graphite starting material and the electrochemical reaction produces reactive chlorine gas (and probably other harmful gaseous products such as ClO2). Additionally, the requirement for a graphite pre-expansion step also adds procedural complexity. For these reasons, this method is not an ideal candidate for industrial scale production of graphene oxide.
US2013/0161199A1 describes a method for the electrochemical production of graphene and graphene oxide by intercalation and exfoliation of graphite starting material. In particular, this document teaches the use of a first bias voltage to effect intercalation of ions from an electrolyte into the graphite layers followed by application of a second (i.e. increased) bias voltage to drive exfoliation of the graphite to form graphene and graphene oxide and the subsequent filtration of the electrolyte to isolate the exfoliated product. Higher bias voltages and higher acidity of the electrolyte are taught to be required to produce graphene oxide via this method (see e.g. paragraph 54). The use of powerful chemical oxidants such as potassium bichromate, permanganic acid and potassium permanganate is also proposed (paragraph 40). Thus, this method has similar drawbacks to the [You 2011] method described above in its requirement for a more procedurally complex two-step intercalation-exfoliation procedure, use of reactive electrolyte materials and a requirement for higher voltages to effect for exfoliation would be arguably more energy consuming than if lower voltages were used and so less desirable from an industrial perspective.
As is evident from the above comments, further methods for the production of graphene oxide/graphite oxide nanoplatelet structures are desired so as to mitigate or obviate one or more of the problems identified above. In particular, methods are desired that produce graphene oxide sheets with a controlled number of layers and flake size.
Advantageously, the methods should be scalable to allow for the production of graphene oxide on a large (preferably industrial) scale. For instance, there is a desire to provide new methods that produce graphene oxide/graphite oxide nanoplatelet structures selectively over other oxidised carbon allotropes, which avoid handling of highly reactive starting materials/products, which are amenable to scale-up to an industrial platform, which are more efficient, reliable, environmentally friendly, provide higher quality material, provide increased yields of material, provide increased oxidation levels, provide larger sheets of material, provide easier isolation of material, which are procedurally simpler and/or which are cheaper than the methods of the prior art.